1. Field of the Invention
The invention relates to a battery characteristic detecting method and a battery characteristic detecting device.
2. Description of the Related Art
If a characteristic change, e.g., a memory effect and degradation, occurs in a battery serving as an electric power source, the ability of the battery is limited. It is therefore important to detect a characteristic change in the battery. For example, Japanese Patent Laid-Open Publication No. 2002-42895 discloses a method for detecting internal resistance in a battery, and detecting whether a memory effect or degradation has occurred in the battery based on the internal resistance. In this method, linear regression of current/voltage characteristics, which are obtained when the battery is being charged/discharged, is performed, and the internal resistance is obtained based on the gradient of the line.
Another method for accurately and easily detecting a degraded condition and estimation of capacity of a sealed lead-acid battery is known from JP 8-222279 A. In this method, an internal resistance of a sealed lead-acid battery is detected and a voltage ΔV in a difference between the variation in the discharge voltage, the variation indicating a difference between the discharge voltage before discharging and the discharge voltage when a prescribed time has elapsed since the start of the discharge, and the product of internal resistance and the prescribed electric current are found. A present battery capacity is estimated from the correlative relationship between the internal resistance and battery capacity in the voltage variation ΔV.
From EP 0 887 654 A2, methods for detecting a working condition of non-aqueous electrolyte secondary batteries are known, which allow easy and accurate determination of the degree of the degradation and remaining capacity of a non-aqueous electrolyte secondary battery by a simple test irrespective of the past charging and discharging history of the battery. In these methods, the degree of degradation of battery is quantitatively determined on the basis of the voltage value in charging or discharging at a constant current, or from an equation with that voltage value as variable.
From JP 2000-121710, a battery control device for back up power supply and a method for diagnosing the deterioration of a secondary battery are known, wherein a battery pack is connected to an equipment body and drives the equipment body with the output of an assembly battery. A battery voltage detection means detects the output voltage of the assembly battery and a single battery for composing it.
Also, a temperature detection means detects the temperature of the assembly battery in the single battery for composing it by a thermistor. A control part operates a charge control means based on information that is detected by a voltage detection means and a temperature detection means and further sends a control signal to the equipment body via a communication means. The control part reports the obtained information to the user of equipment by a reporting means as needed. As the reporting means, for example, a liquid crystal display for displaying information and an LSD, a speaker for generating an alarm sound, and a vibrator for generating vibration are used.
Still another method for detecting a battery pack condition is known from JP 09-113588, wherein this method includes the steps of measuring impedance by a pack battery as a whole when electricity is conducted to the entire battery pack in which a plurality of single batteries are connected to one another in series, determining the life end of the pack battery when this impedance indicates prescribed impedance or higher set on the basis of a reference which is set by using the case that one single battery is in a deteriorated state and all the remaining single batteries are in brand new states in which the capacity of the pack battery is lowest against the identical impedance in a relationship between the previously obtained impedance of the pack battery and a capacity range and determining the normal condition of the pack battery when this impedance is within the prescribed impedance.
JP 5-281 309 A discloses another method and device for detecting deterioration of a lead battery. According to the device of JP 5-281 309 A, a resistor is connected to a sealed lead battery through a switch. When the battery is made to discharge by closing the switch by ≦1 msec. Then, by measuring the difference between the battery voltage before discharge and the battery voltage in a stable state after the discharge and utilizing a strong correlation between the voltage difference and the battery capacity, the capacity of the battery is found from the voltage difference and, when the capacity is lower than a prescribed value, it is determined that the battery is deteriorated. Since the testing time is set at 1 msec, no spark due to the completion of discharge is generated and the safety can be improved even when a tester is disconnected from the battery by mistake.
Under normal use of the battery, however, there are not many chances that a large current flows when the battery is being charged/discharged. Therefore, according to the above-mentioned method, when data is collected in a short time only a small amount of data concerning a region, where a large current flows when the battery is being charged/discharged, can be obtained. Accordingly, the obtainable data are likely to be concentrated around the origin point (the point at which a voltage is “0V” and a current value is “0A”). If linear regression is performed using the data concentrated around the origin point, an internal resistance value cannot be obtained with high accuracy. As a result, it is difficult to detect a memory effect accurately.